Hydrofoil for watercraft with lift generation through air supply of the foil underside

ABSTRACT

A hydrofoil for watercraft, the lift of which can be influenced through the admission or feed of a controllable quantity of air to the subpressure or underpressure region of the foil profile and which possesses air exit openings which extend in the span width direction of the foil. The foil profile possesses an approximately linear mean line, so that with chord-parallel flow there at most occurs an insufficient lift and for generating sufficient lift the under side of the foil profile is supplied with air, the quantity of which can be varied for the purpose of regulating lift.

United States Patent von Schertel [451 July 29, 1975 [54] HYDROFOIL FORWATERCRAFT WITH 3,730,123 5/1973 Lang 114/665 H LIFT GENERATION THROUGHAIR 37321107 6/1973 Miller 137/112 x SUPPLY OF THE FOIL UNDERSIDEFOREIGN PATENTS OR APPLICATIONS [75] Inventor: Hanns von Schertel,Hergiswil, 549,266 /1956 ltaly 114/665 H Switzerland [73] Assignee:Supramar AG, LucerneSwitzerland Primary Blix Assistant Examiner-Barry L.Kelmachter Flledi June 41 1973 Attorney, Agent, or Firm-Werner W.Kleeman [21] Appl. N0.: 366,796

[57] ABSTRACT [52] US. Cl. 114/66.5 H A hydrofoil for watercraft, thelift of which can be in- [51] Int. Cl B63b 1/24 fluenced through theadmission or feed of a controlla- [58] Field of Search 1 14/665 H; blequantity of air to the subpressure or underpressure 137/1 11-1 13 regionof the foil profile and which possesses air exit openings which extendin the span width direction of [56] References Cited the foil. The foilprofile possesses an approximately UNITED STATES PATENTS linear meanline, so that with chord-parallel flow there 2 986 899 6/1961 Schenk Cl111. 137/111 x at most Occurs an insufficient lift and generating3:146:751 9/1964 von SChCl'lCl 114/665 H Sufficient lift the under Sideof the foil Profile is p- 3.335,6s7 8/1967 V011 Schertel 114/665 H pliedwith the q n ity f which can be varied for 3,343,512 9/1967 Rasmussen 1114/665 H the purpose of regulating lift.

3347.197 10/1967 Schercr 114/665 H 3,454,029 7/1969 Frcdd 137/111 x 8Claims, 4 Drawlng Flgures eat/ RE VALVE SERVO/R VALVE 6b COMPRESSORONTROL ADMISSION VALVE VALVE VALVE 0 3b 2/ r 2) C PATEminJuLzsms3.896.752

$HEET 1 m 231 8155 Any 5%? 25 RESERgg/R VALVE COMPRESSOR 6a Kb ADMISSIONag? VALVE .5 5 d 515 519i a CONTROL VA LVE RE VERSING 7 6'ac VALVECONTROL VALVE PATENTEDJULZQISYS 3,896,752

SHEET 2 HYDROFOIL FOR WATERCRAFT WITH LIFT GENERATION THROUGH AIR SUPPLYOF THE FOIL UNDERSIDE BACKGROUND OF THE INVENTION The present inventionrelates to a new and improved construction of hydrofoil for watercraftwhich can attain high speeds free of cavitation.

It is well known that so-called sub-cavitating foils hardly can exceedspeeds of 60 knots, because then it is no longer possible to avoidextensive cavitation. Due to this phenomenon the resistance is markedlyincreased and limitations are placed upon the controllability of lift,for instance by changing the angle of attack or through the action of apivotable foil flap. The stability of a vehicle with completely immersedfoils can be, however, only maintained by automatic variations in thelift, which, for instance, can be controlled by a depth sensor in arestoring sense. A damaging side effect of cavitation resides in theerosion of the material of the foil occurring by virtue of the impact ofthe water which arises upon collapse of the cavitation bubbles.

The problem which is encountered in maintaining the foil free ofcavitation resides in maintaining the excess speeds at the foil profileand the therewith associated pressure drops as small as possible so thatthe absolute pressures remain above the vapor pressure for theprevailing water temperature. This can be realized with the knownprofile contours of small camber (i.e. thin profiles) with asrectangular as possible pressure distribution and small lift coefficient(Ca). However, at speeds exceeding 60 knots there measures lead, on theone hand, to such small profile thicknesses that such no longer arecapable of withstanding the tensile loads and, on the other hand, resultin such small lift coeffi cients that the lift'drag ratios (Cw/Ca)assume impermissibly high values. Owing to the small lift coefficientsthe sensitivity to changes in the flow angle, which occur in the seawayowing to the orbital motion in the water, become extremely large,impairing the seaworthiness of the watercraft.

Attempts have been made to overcome these difficulties through the useof so-called base-ventilated foils. Such possess foil profiles with ablunt rear edge, whereby with the same thickness ratio as a standard,sub-cavitating foil, there occurs a flatter camber of the upper side andtherefore a reduction in the local excess speeds or velocities.Consequently, up to higher speeds than with conventional foils the foilremains cavitation free. In order to reduce resistance the blunt rearedge of the profile is vented via the blunt rear edge of the struts.Such foil is, however subject to dangerous sudden lift collapse effectswhen the infed air penetrates from the rear at the suction side of thefoil profile during pronounced increase of the angle of attack. It isfor this reason that such solution has not found any practicalapplication.

SUMMARY OF THE INVENTION Hence, it is a primary object of the presentinvention to provide a new and improved construction of hydrofoil forwatercraft which is not associated with the aforementioned drawbacks andlimitations of the prior art proposals.

Another and more specific object of the present invention aims atovercoming the above-discussed problems through a completely novelprinciple of generating lift of the hydrofoil so as to extend the pointwhere cavitation occurs to high speeds, to reduce the resistance and todecrease the sensitivity to changes in the flow angle.

Now in order to implement these and still further objects of theinvention, which will become more readily apparent as the descriptionproceeds, the invention contemplates that the foil profile possesses anapproximately linear mean line, so that with flow parallel to the chordof the foil there occurs at best an insufficient lift and for generatingsufficient lift a controllable air quantity is admitted from theatmosphere through exit openings to the underside 'of the profile at itsunder pressure region.

The lift at this approximately symmetrical bi-convex profile thus occurswith chord-parallel flow, due to reduction in the suction force at thefoil underside through the air feed, so that the under pressure over theupper surface, or the difference between the under pressure across theupper surface and the under pressure which still prevails across theunderside of the foil generates the lift. In so doing, the lift can berandomly varied by changing the admitted or infed quantity of air(dosing by means of valves) in a manner such that the lift increaseswith increasing quantity of air and decreases with decreasing quantityof air. The upper surface of the profile can have a small camber with asrectangular as possible pressure distribution, so that only small excessspeeds occur, without the profile thickness becoming insufficientlysmall as concerns its strength characteristics, because the foilunderside which has become at least partially ineffectual by virtue ofthe air feed, approximately doubles the profile thickness with itscamber height.

Further, to additionally prevent the formation of cavitation the foilpreferably contains exit rows at the upper or top surface, to whichthere is then only admitted an automatically controlled quantitiy of airor a quantity of air which is controlled by a sensor when the flow angleincreases.

The advantage of the new technique for generating lift however not onlyresides in the fact that excess speed at the suction side can bemaintained small, notwithstanding a sufficient profile thickness,similar to the situation of the base-ventilated foil, but also thatthere are realized a whole spate of further advantages of notablesignificance, which will be considered hereinafter:

1. Due to the air feed of the underside by means of air admission fromthe atmosphere there occurs a considerable reduction in the profileresistance. This could be proven by tank tests. Consequently, it ispossible, in contrast to the conventional wetted foil sections to reducethe lift coefficient, without being confronted with poor lift/dragratios, whereby again it is possible to approach higher velocityregions. Furthermore, the stresses at the foils remain within tolerablelimits owing to the lower surface loads.

2. As has been established by trials in cavitation tanks at lowercavitation values the lift gradient (dC /da) is reduced by virtue of theair feed and can almost amount to zero with the feed or supply throughexit openings located at the front portion of the profile, that is tosay, the foil hardly responds to changes in the angle of attack. This isso because at the underside the suction force and therefore theoutflowing quantity of air increases with decreasing angle of attack(lift increase) and the lift losses are partially or completelycompensated, whereas with increasing angle of attack the reverse occurs.Hence, there is present an auto matic lift control having the effect ofa reduction in the lift gradient.

The reduction in the lift gradient is of decisive importance for thebehavior in the Seaway, because thereby the influence on lift due to theorbital motion in the waves is lessened or completely overcome. Thechanges in the flow angle of the hydrofoil, arising due to the movementof the water particles, constitutes the greatest problem with respect tothe seaworthiness of the hydrofoil boat or vehicle in a following sea.When the boat has departed from the wave front the lift of the foil bythe upwardly directedcomponents of the orbital velocity is increased, sothat it tends to emerge from the wave valleys in order to then break lowinto the following wave ridge, where the orbital component is downwardlydirected. Although the change in the flow angle at the foil in theSeaway decreases proportional to the velocity, high speed boats owing totheir small lift coefficient, are markedly influenced by the orbitalvelocity, so that the reduction of the lift gradient assumes increasedsignificance with the increase in speed.

In order to compensate changes in lift at conventional foils, whichoccur owing to a change in the flow angle, the foil must be pivotablymounted and automatically controlled with regard to the flow angle suchthat it adjusts itself approximately in the flow direction. Thisconstitutes a considerable expenditure since, apart from the pivotalmounting of the foil, there must be provided a sensor which responds tothe flow direction, and which delivers its commands to a hydraulicactuation mechanism which again brings about pivotal movement of thefoil. The advantages of the foil of this development therefore should bequite clear.

3. Just as the known hydrofoils, the lift of which is generated bycamber of the mean line and inclination of the profile chord at apositive angle with respect to the flow direction, the foil of theinvention with air fed on the underside during approaching the watersurface or level in conventional manner experiences a reduction in liftdue to the surface effect, having a selfstabilizing action. The foilswhich are air'fed at the upper surface of the foil exhibit a small liftincrease as they approach the water surface, so that the immersion depthcontrol is rendered more difficult and the danger of breaking throughthe water surface with complete destruction of lift due to air entryover the entire foil is increased.

The inventive foil is accordingly considerably easier to stabilize thanconventional fully wetted foils and foils which are air-fed at the uppersurface of the foil, owing to its small lift gradient and the morefavorable action of'the surface effect.

Finally, the foil with lift generation through supply at the pressureside generally affords the advantage in contrast to the conventionalfully wetted foils, that the air feed or supply can be simultaneouslyemployed for controlling lift, whereby the difficult pivotal mounting ofthe foil for the purpose of changing the angle of at tack, or pivotablymounted flaps at the foil rear edge and its hydraulic adjustmentmechanism, can be dispensed with.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood and objects other than those set forth above, will becomeapparent when consideration is given .to the following detaileddescription thereof. Such description makes reference to the annexeddrawings wherein:

FIGS. 1 and 2 respectively illustrate embodiments of hydrofoils insectional view with their air channels or,

ducts and exit openings, the air valves with the infeedu lines havingonly been schematically depicted;

FIG. 3 is a plan view of a portion, of the foil depicted in FIG. 2', andg FIG. 4 schematically illustrates in sectional view a regulatingor'control valve with overpressure supply.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now thedrawings, FIGS. I and 2 illustrate embodiments of two foil sectionsequipped with the inventive system for generating lift. The direction offlow I has been illustrated by the depicted arrow of each Figure. InFIG. I the mean line 1 is a straight line having not occur any lift.Lift only then occurswhen air is sucked from the free atmosphere out ofthe air exit opening 3 (i.e. the openings 3a -3c of FIG. I and ;3a-3c ofFIG. 2), throughthe negative pressure at the under side of the foil andthere is thus decreased the magni tude of the negativepressure. The airis delivered to these exit openings 3 by the ductsor channels 4 (Le. theducts 4a-4lc of FIG. I and 4a-4c of FIG. 2) which are connected withcorresponding conduits or channels 5 (ile. channels 5a-5c of FIG. 1 and5a-5c of FIG. 2), here only schematically indicated by a line, at thestruts. These channels 5 are operatively connected by I theschematically depicted regulating or control valve 6 (ie valves 60, 6b,6c of FIG. 1 and fiac, 6b of FIG. 2) with the free atmosphere. Thevalves 6, which also can be arranged directly at the inlet to thechannels 4, regulate the inflow of the quantity of air and arecontrolled in turn by sensors which respond to the movement of thevehicle.

With undisturbed travel speed the foil underside is supplied with aquantity of air which brings about approximately one-half of the maximumlift and which maintains the vehicle weight in equilibrium, so that thelift can be increased by increasing the quantity of air and can bereduced by reducing the quantity of air, in both directions to the sameextent. The maximum lift is attained by the quantity of air whichsaturates the foil (in other words at the point where increasing the airquantity has no further effect), and the smallest lift with shut-off airfeed, whereby the lift with chordparallel flow becomes null.

In the embodiment of FIG. 1 there are provided at the underside of thefoil three exit rows 3a, 3b and 3c, which extend completely or partiallyover the span width of the foil and which receive air via the associatedducts or channels 40, 4b and 4c respectively. The infeed of one-half ofthe air quantity can occur simultaneously through all of the air exitopenings 3a, 3b and 30 or through individual ones thereof or a row,whereby then the other rows can be switched-in for the purpose ofincreasing lift.

The position of the air outflow has an influence upon the lift gradient,that is to say, also upon the lift changes, which occur as a function ofthe changes in the angle of attack. The lift gradient is smaller whenair exit occurs at the front profile underside and is larger when theoutlet openings are located at the rear profile underside. Thisphenomenon can be explained in terms of the pressure changes which occurat the foil profle during variation of the angle of attack and which aremost pronounced at the region of the front edge, so that also at thatlocation the sucked-up air quantities vary most markedly in the sense ofa gradient reduction or compensation of the effect of the angle ofattack.

Since during travel in a following sea there is desired as small aspossible gradient, whereas for a course against the sea the orbitalspeed strives to help the vehicle or craft over the tops of the wavesand therefore,

depending upon wave length, response to angle of attack changes to agreater or lesser extent is desirable, the invention contemplates areversibility of the control from a rear to a front exit and vice versa.Thus, for instance, there can be switched-in the air feed or supplythrough the exit opening 3a for travel directions against the sea andair feed through the opening 30 for travel directions with the sea, andspecifically, with a quantity of air which produces one-half of themaximum lift. To increase the lift there is then switched-in, forinstance, the air exit row 3b, shortly prior to reaching saturationthrough the rows 3a or 30 respectively.

The apparatus for carrying out the reversal or switching operationbetween a front and rear exit row can be constructed, for instance, asdepicted in FIG. 2, such that both rows can be controlled from the sameregulating or control valve 6ac and that an electrically operatedreversing valve 7 which is controlled as a function of the controlstatus or condition selectively releases the flow path to the desiredduct or channel 4a or 4C.

Also by supplying the upper surface of the foil profile by means of anexit row which is situated at the region of the head of the foilprofile, it is possible to attain a reduction in the lift gradient.Trials have however shown that such configuration is practicallynonusable, because there then occur impermissibly high auxiliaryresistances and the lift is markedly increased in an extremely dangerousmanner when approaching the water surface.

For further safeguarding the foil against cavitation and compensatingfor increases in lift, which can arise with large positive flow anglesdue to the orbital motion in the seaway, after the air feed at theunderside or lower side of the profile has already been completelyclosed, there are also provided at the upper surface of the profile airexit openings 3d and 3e to which there is first admitted air when theflow angle or lift is increased. The rear row 3d depicted in theexemplary embodiment of FIG. 1 serves for control purposes, whereas therow 32 serves to counteract cavitation. This row, as taught in mycopending commonly assigned U.S. application, Ser. No. 366,795 filedJune 4, 1973, and entitled Automatic Mechanism For Preventing CavitationAt Air-Fed Hydrofoils And Flow Bodies is arranged at the location wherethere occurs the maximum excess speeds and is connected with anautomatically opening admission valve 8e which opens shortly prior tooccurrence of the cavitation pressure.

The exit row 3d is also preferably placed at a second location of thegreatest excess speed occurring during small angles of attack and, apartfrom the control valve 6a, is also coupled with an admission valve 8daccording to the teaching of the aforementioned application.

In the exemplary embodiment of FIG. 2 the mean line 1 of the foilprofile is slightly domed towards the bottom, so that withchord-parallel flow there occurs a slight negative lift. Consequently,the camber of the upper surface of the foil profile becomes flatterwhich favors extending cavitation into higher speed ranges. Lift alsooccurs in this instance with chord-parallel flow only upon admission ofair to the foil underside. The supply or feed of that quantity of airwhich results in approximately one-half of the maximum lift, occurs inthis case for instance conjointly through the outlets or exits 3a and3b. It should be readily apparent that with the profile form of theembodiment of FIG. 2 air exit rows 3 also can be provided at the uppersurface of the foil corresponding to the arrangement of FIG. I.

In order to counteract the occurrence of cavitation at the undersidewith negative flow angles, there is here depicted an automatic admissionvalve St for the air exit row 30, as taught in the above-mentionedapplication, whereby the exit row 30 is placed at the location ofmaximum excess velocity with negative flow angles.

The maximum lift also can be increased if the exits at the undersidehave air delivered thereto at an excess pressure instead of atatmospheric pressure. The required excess pressure air quantity can begenerated by a blower or compressor 23 (FIG. I) which is coupled withthe drive equipment, or which can be drained off from the last stage ofthe compressor of a gas turbine, if such a propulsion engine isprovided. Because the de livered air pressure of the mentionedcompressors is usually quite high and a pressure of only about I at 14.2lb/in is advantageous for foil feeding, the compressed air is ledthrough a pressure reducing valve 24 and then accumulated in a reservoir25. Since the excess pressure or overpressure feed occurs only briefly,air is supplied intermittently from the reservoir and the airrequirement remains very small.

Any of the outlet rows 3 can be fed with overpressure, but with anarrangement of a number of exit rows one behind another, preferably onlya row placed in front of another is charged with overpressure i.e. row3b or/and 3c in the Figures. In FIG. 1 exit row 3b is fed withoverpressure as an example. In this case the air inlet valve 6b admitscompressed air only, but is operated such that it opens first when thefoil is almost saturated by the rows situated therebehind. In adifferent technique with a lower consumption of compressed air thecontrol valve 6b can be replaced by the valve which is depicted in FIG.4 and which is then numbered 22 in FIG. 1. This valve is constructed insuch a way that in the opening direction during a first phase it admitsair at atmospheric pressure until a valve outlet area is attained bywhich the foil is almost saturated, and then during a second phaseadmits a quantity of air which is at excess pressure.

FIG. 4 schematically illustrates an example of such control orregulating valve. In the valve housing 9 there is arranged a valve plate10 which is actuated by the sensors and which valve plate 10 controlsthe throughpassage or flow in the foil from the chamber 11 to theassociated channel or duct 4. This chamber receivesits air inflow orsupply from the atmosphere by the check valve 12. The valve plate 10 iscoupled with a slide valve means 13/14 which controls throughflow froman annular chamber 15, which is charged by component 19 with excess oroverpressure p, to the compartment 11. When the valve plate is openedfor the most part and the slide element 13 has moved past the lower edgeof the annular compartment then compressed air can flow into thecompartment or chamber 11. As soon as the pressure in this compartmenthas risen by a small amount then the spring 20 closes the check valve 12and the associated channel 4 is now charged with overpressure for suchtime until the slide valve again closes the connection from the excesspressure infeed 19 to the compartment 11.

It should be readily understood that a random number of air exit rowscan be provided at the profile underside and the profile upper side andthat also the exit openings at the upper side can be charged with excesspressure in the described manner.

By virtue of an advantageous construction of the air exit openings,which can be in the form of bores or slots, it is possible to influencethe action of the feed or supply and the resistance. Advantageousconstructional forms have been depicted in FIGS. 2 and 3. As can berecognized by referring to FIG. 2 and also FIG. 1 these bores areinclined towards the rear. The exit 31) of FIG. 2 and FIG. 3 has ashell-like milled portion 16 behind the bore in order to obtain a smoothair outflow tangentially with respect to the foil surface. Even moreadvantageous is the outflow or exit 3a, at which there is provided agroove 17 which extends in the span width direction, the front portionof which possesses'a low almost vertical wall 18 at which the bores openand the rear portion of which merges in a continuous curve into theprofile contour. With this constructional embodiment the air does notdepart in individual jets, as is the case for exit 31), rather it canexpand within the groove transverse to the direction of travel, so thatthe air leaves the groove in the form of a closed veil or film, therebyattaining the lowest resistance. In the event that a slot is provided asthe outlet or exit, then the outlet mouth is formed in analogous manner.The described constructions for the air exits are employed both at theunderside as well as the upper side of the foil profile.

In order to increase the suction force at the air exit rows at theunderside of the foil profile, it is possible, as depicted in FIG. 1, toarrange in front of the exit rows a wedge or stepped portion 21 whichextends at least partially over the span width and over the contour ofthe foil profile, this wedge or stepped portion extending with a bluntportion towards the rear and hav ing a flat starting portion extendingtowards the front.

The foil which is designed according to the invention, for the reasonsto be explained hereinafter, is particularly suitable for vehiclesoperating at high speeds. Apart from the fact that the foil can attainhigh speeds of velocities without cavitation, it has been found throughexperimentation that the air feed or supply with respect to theoccurring resistance becomes that much more favorable the greater thespeed. Furthermore, there increases the effectiveness of the aircontrol, which can be expressed in terms in relation to the ratio AC /Ci.e. the attainable lift variations of the average lift, with speed,because the value of C becomes smaller with increasing speed.

i The mean line of the foil profile, which according to the inventionand as a prerequisite for a weakly cambered upper surface, at mostgenerates an insufficient lift, can possess a random curved shape, forinstance can be approximately parabolically curved or curvedin aS-shape. For high velocities or speeds exceeding 60. knots the thicknessratio of the profile is considerably smaller than illustrated in thedrawings.

It should be further understood that the novel foil of this developmentalso can be employed for other purposes than for hydrofoil boats, forinstance as a starting aid for hydroplanes.

While there is shown and described present preferred embodiments of theinvention, it is to be distinctly understood that the invention is notlimited thereto, but may be otherwise variously embodied and practicedwithin the scope of the following claims.

What is claimed is:

1. System for generating lift at a hydrofoil for watercraft, thehydrofoil having air exit openings extending in the span width directionthereof, the lift of said hydrofoil being influenced by the admission ofa controllable quantity of air to the negative pressure region of thehydrofoil through said air exit openings, the improvement comprisingsaid hydrofoil profile having an approximately linear mean line suchthat chord parallel flow generates insufficient lift to support theweight of said watercraft, means for generating sufficient lift tosupport the weight of said watercraft, said lift generating meanscomprising said air exit openings being located on the underside of thefoil profile, a wedge positioned forwardly of said air exit openings andextending to a point below the profile of said hydrofoil, said wedgehaving a rearwardly directed blunt edge, means for continuouslysupplying a quantity of air to said air exit openings and means forvarying the quantity of air admitted to said air exit openings forregulating the lift.

2. The system as defined in claim 1., wherein the air supplying meanssupplies the underside of the hydrofoil, at undisturbed cruising speed,with a quantity of air equal to approximately one-half of the airquantity i supplied for maximum lift.

matic admission valve for feeding air to such air exit openings when thevapor pressure of the water has almost been attained.

5. The system as defined in claim 1, wherein a groove extends at leastover a part of the span width of the foil at least some of the air exitopenings communicating with said groove, the front portionof the groovebeing formed by an almost vertical wall whereas the rear por-,

tion extends into the contour of the profile in a continuous curve.

6. The system as defined in claim 1, further including means fordelivering air at excess pressure to the air exit openings and at leastone valve means for regulating the air admission, constructed such thatin the opening direction during a first phase said valve means,

admits air under atmospheric pressure until a valve outlet area isattained by which the foil is almost saturated, and during a secondphase said valve means admits air which is at excess pressure.

7. The system as defined in claim 1 wherein said means for continuouslysupplying air to said air exit openings includes means for supplyingatmospheric air and means for supplying air at excess pressure, andfurther including valve means operatively connecting said excesspressure air supplying means and said atmospheric air supplying meanswith said air exit openings for delivering atmospheric air to said airexit openings until the hydrofoil is almost saturated and thereafterplate has almost attained its maximum open position. :0:

1. System for generating lift at a hydrofoil for watercraft, thehydrofoil having air exit openings extending in the span width directionthereof, the lift of said hydrofoil being influenced by the admission ofa controllable quantity of air to the negative pressure region of thehydrofoil through said air exit openings, the improvement comprisingsaid hydrofoil profile having an approximately linear mean line suchthat chord parallel flow generates insufficient lift to support theweight of said watercraft, means for generating sufficient lift tosupport the weight of said watercraft, said lift generating meanscomprising said air exit openings being located on the underside of thefoil profile, a wedge positioned forwardly of said air exit openings andextending to a point below the profile of said hydrofoil, said wedgehaving a rearwardly directed blunt edge, means for continuouslysupplying a quantity of air to said air exit openings and means forvarying the quantity of air admitted to said air exit openings forregulating the lift.
 2. The system as defined in claim 1, wherein theAir supplying means supplies the underside of the hydrofoil, atundisturbed cruising speed, with a quantity of air equal toapproximately one-half of the air quantity supplied for maximum lift. 3.The system as defined in claim 1, wherein the underside of the hydrofoilis provided with a plurality of rows of air exit openings arranged inspaced relation transversely of the span width direction of saidhydrofoil and means for selectively switching the air supply between aforwardly and rearwardly disposed row of air exit openings in order tochange the lift gradient.
 4. The system as defined in claim 1, whereinair exit openings provided at locations of the maximum prevailing excessspeeds, and means including an automatic admission valve for feeding airto such air exit openings when the vapor pressure of the water hasalmost been attained.
 5. The system as defined in claim 1, wherein agroove extends at least over a part of the span width of the foil atleast some of the air exit openings communicating with said groove, thefront portion of the groove being formed by an almost vertical wallwhereas the rear portion extends into the contour of the profile in acontinuous curve.
 6. The system as defined in claim 1, further includingmeans for delivering air at excess pressure to the air exit openings andat least one valve means for regulating the air admission, constructedsuch that in the opening direction during a first phase said valvemeans, admits air under atmospheric pressure until a valve outlet areais attained by which the foil is almost saturated, and during a secondphase said valve means admits air which is at excess pressure.
 7. Thesystem as defined in claim 1 wherein said means for continuouslysupplying air to said air exit openings includes means for supplyingatmospheric air and means for supplying air at excess pressure, andfurther including valve means operatively connecting said excesspressure air supplying means and said atmospheric air supplying meanswith said air exit openings for delivering atmospheric air to said airexit openings until the hydrofoil is almost saturated and thereafterdelivering excess pressure air to said air exit openings.
 8. The systemas defined in claim 7, wherein said valve means comprises a compartmentcontaining a valve plate for regulating a throughpassage from thecompartment to the hydrofoil, a check valve having an open and a closedposition and connecting said compartment with said means for supplyingatmospheric air, a slide valve means operatively coupled with said valveplate and connecting said compartment with said means for supplyingexcess pressure air for admitting excess pressure air to saidcompartment when the valve plate has almost attained its maximum openposition.